Any number can be assigned to enum member type

[rlz]

Search Terms:
enum assign number

Code

enum X {
  A, B
}

const x: X.A = 5;

const y: X.A = X.B;

Expected behavior:
2 errors: when assigning number literal and wrong enum member.

Actual behavior:
Typescript allows assigning any number to specific member type.

Playground Link:
http://bit.ly/2CZsUrK

[DanielRosenwasser]

I was very close to marking this as a duplicate (mostly with #6131 in mind, but also sort of #8020 - see #8020 (comment)).

The key thing here is that we're working with an individual enum member type, not just an enum type.

[rlz]

Yes, I expected member type is 0 in this case, so I should be able to assign 0 but not 5.

[RyanCavanaugh]

So we'd allow e.g. const x: X.A = 5 + 0 but not = 5 ?

[RyanCavanaugh]

The key thing here is that we're working with an individual enum member type, not just an enum type.

The problem here is that X is just an alias for X.A | X.B, so any argument about assignability to X.A applies equally to assignability to X.

Given the back compat constraints and unlikelihood of finding a bug in userland, keeping the current behavior seems like the best choice of action.